The invention relates to a display device, in particular for a vehicle, having a screen which has a liquid crystal cell, with the liquid crystal cell having a front and a rear cell wall and a liquid crystal substance arranged in the cell space between these cell walls, and having a heating apparatus by means of which the display device can be heated.
In liquid crystal cells which, at least at times, are operated at low ambient temperatures, for example as is the case with displays in vehicles, the problem occurs that the switching times of the liquid crystal cells become undesirably long at low temperatures, so that displays equipped with them are subject to considerable inertia. In order to overcome this undesirable situation, it is known for a light box to be provided, which is used to illuminate a liquid crystal cell in a liquid crystal display and has a heating wire. The heating wire heats the liquid crystal cell and maintains it at a temperature level which, despite low ambient temperatures, allows the liquid crystal display to be operated with the necessary short switching times. Owing to the relatively long distance between the heating wire and the liquid crystal cell, this necessitates a large amount of power being supplied and the heating wire being strongly heated in order to allow the heating required for the liquid crystal cell to be produced. Furthermore, owing to its arrangement in the light box and its necessary high level of thermal emission, the heating wire heats the entire display unit, in an undesirable manner.
The invention is based on the object of designing a display device having a liquid crystal cell such that the display unit can be operated reliably, and with only short switching times, even at low ambient temperatures.
This object is achieved according to the invention in that the heating apparatus is arranged in the cell space of the liquid crystal cell directly or indirectly on the front and/or on the rear cell wall, and in that the heating apparatus is operated with a pure AC voltage.
By virtue of such a design, the liquid crystal substance is heated directly and very quickly, without any undesirable power losses occurring as the heat is passed on. The heating power produced by the heating apparatus may thus be particularly low. This prevents parts of the display device which must not be heated from being heated undesirably. Furthermore, the heating apparatus can easily be manufactured at the same time that the liquid crystal cell is produced, without any additional assembly measures being required. Operation of the heating apparatus with a pure AC voltage ensures that the display device has a long life and is operationally reliable. If a DC voltage is applied to a heater in a liquid crystal cell which can be heated, this results in a potential difference between the contact points; the liquid crystal layer that is in direct contact with the heating apparatus is then subject to a permanent DC voltage. Depending on the magnitude of the DC voltage or of the DC voltage component of an AC voltage, this leads to the liquid crystal decomposing irreversibly in the short term or medium term, and thus reduces the life and reliability of the display apparatus to an unacceptable extent. By using a pure AC voltage according to the invention to actuate the heating apparatus, with a low DC voltage component being allowed for the purposes of tolerances that cannot be avoided with reasonable complexity, the decomposition effects described above are effectively counteracted, so that the display apparatus according to the invention is distinguished by its long life and the clarity of the display.
As a result of the direct association of the heating apparatus with the liquid crystal substance and the lack of any thermally insulating layer, the display device heating-up and switching times are particularly short. Thusxe2x80x94and owing to the high operational reliability described abovexe2x80x94the display device according to the invention is also particularly suitable for a tachometer display in a motor vehicle. In the case of a tachometer, short response times, immediate operating capability and unlimited reliability are particularly important, since it is of major importance to traffic safety to display the driving speed.
The display unit has a particularly simple and economic configuration if the liquid crystal cell is, preferably, a TN or STN cell.
The production costs of the display unit can advantageously be reduced if the heating apparatus has an electrically conductive layer. When current flows, this layer can then act as a resistance heater. It would be feasible for the electrically conductive layer to be arranged immediately adjacent to the liquid crystal substance.
However, for simple liquid crystal circuitry with a power consumption that is as low as possible, it is particularly advantageous for the electrically conductive layer to be arranged between a front electrode and/or a rear electrode of the liquid crystal cell and the front and/or rear cell wall.
The production of the liquid crystal cell is even further simplified, if, according to another advantageous development of the invention, the electrically conductive layer is arranged in the same plane as a front electrode and/or rear electrode of the liquid crystal cell. The electrically conductive layer can thus be applied, together with an electrode of the liquid crystal cell, in one operation.
It would be feasible for the electrically conductive layer to have a large area; however, the heating effect is particularly good if the electrically conductive layer has a meandering profile. It is also feasible for the electrically conductive layer to be arranged only in those areas in which there are switchable display elements. The use of electrical power required for heating can thus be further reduced, since this avoids heating areas of the liquid crystal cell which are not switched, and thus where there is no negative influence from low ambient temperatures.
Particularly in the case of dot-matrix displays using a liquid crystal cell, it is normal for the liquid crystal cell to have a front electrode and a rear electrode each having families of mutually parallel electrode strips, with the electrode strips of the front electrode and rear electrode being arranged such that they cross over one another. In order to achieve a good heating effect, covering an area, with such liquid crystal cells, it is particularly advantageous for the electrode in whose plane the electrically conductive layer is arranged to be in the form of strips, and for elongated sections of the electrically conductive layer to be arranged in the spaces between the strips.
In order to ensure that the elongated sections of the electrically conductive layer are connected in a manner which does not interfere with the contact with the electrodes that are in the form of strips, plated-through contacts, which connect the plane of the front electrode and the plane of the rear electrode, preferably electrically connect the elongated sections of the electrically conductive layer to one another by means of connecting sections.
The electrodes in liquid crystal cells are normally composed of indium tin oxide. The production of a display device according to the invention is thus considerably simplified if the electrically conductive layer is composed of indium tin oxide, so that no additional material need be handled and processed during the production of the liquid crystal cell.
According to another advantageous development of the invention, the functional reliability of the display device is increased if the electrically conductive layer is covered with an electrical insulation layer. The insulation layer is advantageously composed of glass. This may be applied, sintered and fused in as a glass powder.
According to an advantageous development of the invention, the AC voltage which operates the heating apparatus can be applied directly to the electrically conductive layer, in order to keep the power losses as low as possible.
Particularly if the screen is back-lit, it is especially advantageous for the electrically conductive layer to be translucent. A very large proportion of the light emitted from a light source that is used for illumination is thus used without any significant light losses or non-uniform illumination of the screen occurring as would be the case, for example, if heating wires were used, which lead to the formation of shadows.
If, according to an advantageous development of the invention, the front and/or the rear cell wall is a glass plate, then this not only allows the screen to be back-lit (if both cell walls are glass plates), but also allows the liquid crystal cell to be produced easily and economically. Liquid crystal screens may also be illuminated by ambient lighting and/or floodlighting that is incident on them from the front. A reflector arranged in a rear area or behind the liquid crystal cell is required for this purpose, which deflects the incident light and makes the actuated areas of the liquid crystal cell visible to a viewer. It is thus particularly advantageous if the heating apparatus reflects light, so that it can carry out the function of the reflector mentioned above.
According to an advantageous development of the invention, the heating apparatus is a transflector which reflects light that is incident on the front face of the heating apparatus and transmits light that is incident on the rear face. Thus, if the surroundings are sufficiently bright, the display unit can be read without any additional light source while, if the surroundings are dark, a light source arranged behind the liquid crystal cell can be switched on in order to make the display visible.
It is particularly advantageous for the screen to have two liquid crystal cells arranged one behind the other, thus allowing compensation for any undesirable coloring of the liquid crystal cell. To do this, it is sufficient for one of the cells to be actuated actively and for the other cell to be operated purely passively, with both cells containing the same liquid crystal substance. However, it is also feasible to use two active cells connected one behind the other, thus allowing superimposed information to be displayed.
Both liquid crystal cells are preferably TN or STN cells. The need for color compensation occurs in particular with STN cells. Two STN cells arranged one behind the other, with one of the cells being operated actively and the other passively, are known by the name DSTN cell.
It would also be feasible for a heating apparatus to be provided in only one of the liquid crystal cells arranged one behind the other. In order to allow the liquid crystal substance in both cells to be heated, a large amount of heating power is then required, however. Furthermore, that liquid crystal substance which is further away from the heating apparatus will be at a lower temperature level than the liquid crystal substance directly associated with the heating apparatus. The non-uniform heating of the two substances can lead to additional, undesirable color effects. It is thus particularly advantageous for each of the liquid crystal cells to have a heating apparatus.
It would be feasible to use the entire area of the liquid crystal cell as a display area for a display. However, in many cases, only part of the area of the liquid crystal cell is envisaged as a display area, and the other areas of the cell are covered by a panel. It is then advantageousxe2x80x94without interfering with the capability to identify the displayxe2x80x94for the heating apparatus to be arranged outside the display area of the liquid crystal cell.
According to another advantageous development of the invention, a temperature sensor is arranged in the cell space of the liquid crystal cell. This makes it simple to use an external electronic circuit to switch the heating apparatus on and off as required, depending on the temperature conditions in the liquid crystal cell.
In a DSTN cell that has two liquid crystal cells optically arranged in series, one of the liquid crystal cells may be provided as an active cell for producing a display, and the other liquid crystal cell as a passive cell for color compensation. In this case, according to another development of the invention, it is particularly advantageous for the screen to have a DSTN cell with an active liquid crystal cell and with a passive liquid crystal cell, with the heating apparatus being arranged in the passive liquid crystal cell. This ensures that the heating apparatus will not have any disturbing influences on the electrical actuation of the liquid crystal cell used for display purposes.
The passive liquid crystal cell in this case preferably has a front electrode and a rear electrode, and the front electrode and the rear electrode can have a voltage applied to them in such a manner that either light characters against a dark background (negative display) or dark characters against a light background (positive display) can be displayed on the screen. The type of display which can be read better in any situationxe2x80x94depending, for example, on the ambient brightnessxe2x80x94can thus be selected without any additional physical complexity.
As a rule, when producing an AC voltage, it is impossible to avoid at least a low DC voltage component, without major technical complexity. According to one advantageous development of the invention, the display apparatus is particularly economic and nevertheless precise, and has a long life, if the AC voltage that operates the heating apparatus has a DC voltage component of no more than 100 mV. This low DC voltage component is too small to cause any undesirable decomposition of the liquid crystal substance.